1. a key expression has changed since the index was created. For
example, if you had an index on LAST and changed the width of the
field from 20 bytes to 25 bytes, and then tried to use the old index,
you would get this error.

2. an index was created on an expression which has a variable length
(e.g., TRIM(last)). The key expression must always return a fixed
length.

Action: Use PADR() to ensure that the key expression is always the
same length.

In other hand, TAG keyword indicate a CDX type indexing; may be an incompatibility between Clipper and Harbour implementing this type indexes.

If possible, you may try indexing without TAG clause.

Bad news is: Harbour compiled applications can successfully open and use database and index files built by Clipper compiled programs; but the inverse isn't correct: Clipper compiled applications may or may not successfully open and use database and index files built by Harbour compiled programs

This is because Harbour have too many extensions and expansions regarding Clipper; so Clipper compiled program may doesn't recognize these extra features.

My suggestion is convert your Clipper applications as soon as possible to Harbour